1. Field of the Invention
The present invention relates to a method and composition employing calcium phosphite as an ultraviolet light stabilizer as well as a plate-out inhibitor for PVC.
2. Discussion of Background References
Nearly all synthetic polymers are susceptible to the degrading effects of mechanical shear, heat, ultraviolet light and oxygen. Manifestations of degradation are discoloration, loss of gloss, surface crazing, chalking and loss of desirable physical properties. Without suitable protection, even those polymers which survive processing can display useful lifetimes measured only in weeks or months. In the case of polyvinyl chloride (PVC), discoloration becomes aesthetically unacceptable during processing even though physical properties may not be seriously affected. A broad variety of stabilizers have been developed to control the degradation process so that, with proper selection, polymer lifetimes as long as 20 years may be achieved.
The polyolefins (polypropylene, polyethylenes, polystyrene, etc.) degrade by a free radical oxidative process initiated by the heat and mechanical shear of processing and by ultraviolet (UV) light. Hindered phenolic antioxidants and organophosphites provide adequate protection during processing and indoor applications. In outdoor applications, however, these additives are consumed too rapidly to provide adequate protection. Hydroxybenzoate esters and hindered amines, which function as catalytic free radical terminators, provide good outdoor protection particularly in combination with phosphites and certain other co-additives.
The degradation of polyvinyl chloride (PVC) is more complicated. The heat and shear of processing initiates an autocatalytic dehydrochlorination which leads to highly colored, conjugated unsaturation. This causes the color of the plastic to change from white to yellow through tan, brown, reddish-brown, and finally black. This discoloration precedes any measurable change in physical properties. Later, free radical oxidation causes loss of gloss, chalking and deterioration of physical properties. The degradation of PVC is further described by Summers et al,, The Chemical Mechanisms of Outdoor Weathering in Polyvinyl Chloride, J. Vinyl Tech., Vol. 5, No. 3, Sept. 1983.
Functional PVC products are compounded systems; virgin resin is never used alone. There are two general classes of polyvinyl chloride; rigid and flexible.
Rigid PVC contains, in addition to the resin, a heat stabilizer, a lubricant, usually a processing aid, usually an impact modifier, usually a light (UV) stabilizer, and various fillers and pigments. All products for exterior use (house siding, window components, gutters, pipe, etc.) must contain a UV stabilizer. A heat stabilizer solves a different problem that does a light stabilizer. A heat stabilizer is intended to alleviate problems caused by heat during manufacture or, for example, by temperatures caused by exposure to sunlight. A light stabilizer is intended to alleviate problems caused by long term exposure to the ultraviolet radiation in sunlight. In rigid polyvinyl chloride, tin mercaptides are superior heat stabilizers but contribute little to light stability. Processing aides such as lubricants, to reduce melt viscosity, and acrylics, to control flow of polymer melt, also contribute to stability. In outdoor applications, such as house siding and gutters, titanium dioxide effectively controls light-initiated degradation whereas the polyolefin light stabilizers are relatively ineffective. Substituted benzotrizoles are also occasionally used as UV stabilizers in PVC. However, it would be desirable to find alternatives for controlling light-initiated degradation.
Flexible PVC contains greater than about 25% by weight of a plasticizer in addition to the other additives mentioned above. It is used in automobile upholstery, wall covering, jacketing for electrical wire and cable etc. In flexible polyvinyl chloride, fatty acid soaps of cadmium, barium, zinc and calcium in combination with phosphites, will provide good protection against discoloration. Flexible PVC usually does not contain a UV stabilizer, although certain products (such as automobile upholstery) do.
Chemical and Process Technology Encyclopedia, pp. 1057-58 (D. M. Constantine ed. 1974) (McGraw-Hill) describes a variety of PVC stabilizers. The stabilizers includes lead compounds, barium and cadmium stabilizer systems, barium compounds, cadmium compounds, zinc compounds, calcium compounds, organic phosphites, polyhydric alcohols, nitrogenous compounds, epoxy plasticizers and organotin compounds.
U.S. Pat. No. 4,221,687 to Minagawa, et al. discloses the use of a calcium salt plus a 1,3-diketone compound as a heat stabilizer, for vinyl chloride polymer compositions to prevent yellowing during fabrication. The calcium salts CaO.CaHPO.sub.3 and 3Ca(OH).sub.2.CaHPO.sub.3 are disclosed in a list of metal basic inorganic acid salts.
U.S. Pat. No. 2,579,572 to Hendricks discloses a vinyl chloride resin composition stable against the action of light and containing from 0.5 to 10 parts by weight of sodium phosphite or potassium phosphite. In addition to the vinyl resin and phosphite salt components, the composition may also contain plasticizers, coloring and modifying agents and, if desired, other stabilizing agents. In particular, Hendricks discloses forming a sodium or potassium phosphite intermediate product containing a lubricant soap such as, for instance, barium ricinoleate.
U.S. Pat. No. 2,604,459 to Jankowiak discloses stabilization of polymers comprising vinyl or vinylidene chlorides against heat and light. It also discloses the amelioration of the effects of inorganic impurities, such as iron, which make the polymer more susceptible to thermal decomposition. The disclosed process involves contacting the polymer with an acidic (adjusted to acidic pH with mineral acid) solution of alkali metal phosphates, polyphosphates or phosphites and drying the additives onto the polymer.
However, none of the above references disclose the use of calcium phosphite as a light stabilizer.
Another problem in polyvinyl chloride processing is the existence of residue from polyvinyl chloride extruding and molding. This residue contaminates subsequent polyvinyl chloride manufacture and is known in the art as plate out. None of the above references relate to this problem nor teach a method of preventing the same.
Furthermore, there is an ample supply of calcium phosphite-containing material provided as a by-product of sodium hypophosphite manufacture. It would be desirable to find a valuable use for this material.